Method for fracturing and forming a pattern using curvilinear characters with charged particle beam lithography

ABSTRACT

In the field of semiconductor production using shaped charged particle beam lithography, a method and system for fracturing or mask data preparation or proximity effect correction is disclosed, wherein a series of curvilinear character projection shots are determined for a charged particle beam writer system, such that the set of shots can form a continuous track, possibly of varying width, on a surface. In some embodiments, characteristics of the continuous track will be within a predetermined tolerance.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/618,722 entitled “Method For Fracturing and Forming a Pattern UsingCurvilinear Characters With Charged Particle Beam Lithography”, filed onNov. 14, 2009, which 1) is a continuation-in-part of U.S. patentapplication Ser. No. 12/603,580 entitled “Method For Fracturing APattern For Writing With A Shaped Charged Particle Beam Writing SystemUsing Dragged Shots”, filed on Oct. 21, 2009; and 2) claims priorityfrom U.S. Provisional Patent Application Ser. No. 61/237,290 filed Aug.26, 2009, entitled “Method and System For Manufacturing a Surface UsingCharged Particle Beam Lithography”; all of which are hereby incorporatedby reference for all purposes.

BACKGROUND OF THE DISCLOSURE

The present disclosure is related to lithography, and more particularlyto the design of a charged particle beam writer system and methods forusing the charged particle beam writer system to manufacture a surfacewhich may be a reticle, a wafer, or any other surface.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, optical lithography may be used to fabricate thesemiconductor devices. Optical lithography is a printing process inwhich a lithographic mask or photomask manufactured from a reticle isused to transfer patterns to a substrate such as a semiconductor orsilicon wafer to create the integrated circuit. Other substrates couldinclude flat panel displays or even other reticles. Also, extremeultraviolet (EUV) or X-ray lithography are considered types of opticallithography. The reticle or multiple reticles may contain a circuitpattern corresponding to an individual layer of the integrated circuit,and this pattern can be imaged onto a certain area on the substrate thathas been coated with a layer of radiation-sensitive material known asphotoresist or resist. Once the patterned layer is transferred the layermay undergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Eventually, a combination of multiplesof devices or integrated circuits will be present on the substrate.These integrated circuits may then be separated from one another bydicing or sawing and then may be mounted into individual packages. Inthe more general case, the patterns on the substrate may be used todefine artifacts such as display pixels or magnetic recording heads.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, maskless direct write may also be used to fabricatethe semiconductor devices. Maskless direct write is a printing processin which charged particle beam lithography is used to transfer patternsto a substrate such as a semiconductor or silicon wafer to create theintegrated circuit. Other substrates could include flat panel displays,imprint masks for nano-imprinting, or even reticles. Desired patterns ofa layer are written directly on the surface, which in this case is alsothe substrate. Once the patterned layer is transferred the layer mayundergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Some of the layers may be written usingoptical lithography while others may be written using maskless directwrite to fabricate the same substrate. Eventually, a combination ofmultiples of devices or integrated circuits will be present on thesubstrate. These integrated circuits are then separated from one anotherby dicing or sawing and then mounted into individual packages. In themore general case, the patterns on the surface may be used to defineartifacts such as display pixels or magnetic recording heads.

Two common types of charged particle beam lithography are variableshaped beam (VSB) and character projection (CP). These are bothsub-categories of shaped beam charged particle beam lithography, inwhich a precise electron beam is shaped and steered so as to expose aresist-coated surface, such as the surface of a wafer or the surface ofa reticle. In VSB, these shapes are simple shapes, usually limited torectangles of certain minimum and maximum sizes and with sides which areparallel to the axes of a Cartesian coordinate plane, and triangles withtheir three internal angles being 45 degrees, 45 degrees, and 90 degreesof certain minimum and maximum sizes. At pre-determined locations, dosesof electrons are shot into the resist with these simple shapes. Thetotal writing time for this type of system increases with the number ofshots. In CP charged particle beam lithography, there is a stencil inthe system that has in it a variety of apertures or characters which maybe rectilinear, arbitrary-angled linear, circular, nearly circular,annular, nearly annular, oval, nearly oval, partially circular,partially nearly circular, partially annular, partially nearly annular,partially nearly oval, or arbitrary curvilinear shapes, and which may bea connected set of complex shapes or a group of disjointed sets of aconnected set of complex shapes. An electron beam can be shot through acharacter on the stencil to efficiently produce more complex patterns onthe reticle. In theory, such a system can be faster than a VSB systembecause it can shoot more complex shapes with each time-consuming shot.Thus, an E-shaped pattern shot with a VSB system takes four shots, butthe same E-shaped pattern can be shot with one shot with a characterprojection system. Note that VSB systems can be thought of as a special(simple) case of character projection, where the characters are justsimple characters, usually rectangles or 45-45-90 triangles. It is alsopossible to partially expose a character. This can be done by, forinstance, blocking part of the particle beam. For example, the E-shapedpattern described above can be partially exposed as an F-shaped patternor an I-shaped pattern, where different parts of the beam are cut off byan aperture. This is the same mechanism as how various sized rectanglescan be shot using VSB. In this disclosure, partial projection is used tomean both character projection and VSB projection.

As indicated, in optical lithography the lithographic mask or reticlecomprises geometric patterns corresponding to the circuit components tobe integrated onto a substrate. The patterns used to manufacture thereticle may be generated utilizing computer-aided design (CAD) softwareor programs. In designing the patterns the CAD program may follow a setof pre-determined design rules in order to create the reticle. Theserules are set by processing, design, and end-use limitations. An exampleof an end-use limitation is defining the geometry of a transistor in away in which it cannot sufficiently operate at the required supplyvoltage. In particular, design rules can define the space tolerancebetween circuit devices or interconnect lines. The design rules are, forexample, used to ensure that the circuit devices or lines do notinteract with one another in an undesirable manner. For example, thedesign rules are used so that lines do not get too close to each otherin a way that may cause a short circuit. The design rule limitationsreflect, among other things, the smallest dimensions that can bereliably fabricated. When referring to these small dimensions, oneusually introduces the concept of a critical dimension. These are, forinstance, defined as the smallest width of a line or the smallest spacebetween two lines, those dimensions requiring exquisite control.

One goal in integrated circuit fabrication by optical lithography is toreproduce on the substrate the original circuit design by use of thereticle. Integrated circuit fabricators are always attempting to use thesemiconductor wafer real estate as efficiently as possible. Engineerskeep shrinking the size of the circuits to allow the integrated circuitsto contain more circuit elements and to use less power. As the size ofan integrated circuit critical dimension is reduced and its circuitdensity increases, the critical dimension of the circuit pattern orphysical design approaches the resolution limit of the optical exposuretool used in optical lithography. As the critical dimensions of thecircuit pattern become smaller and approach the resolution value of theexposure tool, the accurate transcription of the physical design to theactual circuit pattern developed on the resist layer becomes difficult.To further the use of optical lithography to transfer patterns havingfeatures that are smaller than the light wavelength used in the opticallithography process, a process known as optical proximity correction(OPC) has been developed. OPC alters the physical design to compensatefor distortions caused by effects such as optical diffraction and theoptical interaction of features with proximate features. OPC includesall resolution enhancement technologies performed with a reticle.

OPC may add sub-resolution lithographic features to mask patterns toreduce differences between the original physical design pattern, thatis, the design, and the final transferred circuit pattern on thesubstrate. The sub-resolution lithographic features interact with theoriginal patterns in the physical design and with each other andcompensate for proximity effects to improve the final transferredcircuit pattern. One feature that is used to improve the transfer of thepattern is a sub-resolution assist feature (SRAF). Another feature thatis added to improve pattern transference is referred to as “serifs”.Serifs are small features that can be positioned on a corner of apattern to sharpen the corner in the final transferred image. It isoften the case that the precision demanded of the surface manufacturingprocess for SRAFs is less than that for patterns that are intended toprint on the substrate, often referred to as main features. Serifs are apart of a main feature. As the limits of optical lithography are beingextended far into the sub-wavelength regime, the OPC features must bemade more and more complex in order to compensate for even more subtleinteractions and effects. As imaging systems are pushed closer to theirlimits, the ability to produce reticles with sufficiently fine OPCfeatures becomes critical. Although adding serifs or other OPC featuresto a mask pattern is advantageous, it also substantially increases thetotal feature count in the mask pattern. For example, adding a serif toeach of the corners of a square using conventional techniques adds eightmore rectangles to a mask or reticle pattern. Adding OPC features is avery laborious task, requires costly computation time, and results inmore expensive reticles. Not only are OPC patterns complex, but sinceoptical proximity effects are long range compared to minimum line andspace dimensions, the correct OPC patterns in a given location dependsignificantly on what other geometry is in the neighborhood. Thus, forinstance, a line end will have different size serifs depending on whatis near it on the reticle. This is even though the objective might be toproduce exactly the same shape on the wafer. These slight but criticalvariations are important and have prevented others from being able toform reticle patterns. It is conventional to discuss the OPC-decoratedpatterns to be written on a reticle in terms of main features, that isfeatures that reflect the design before OPC decoration, and OPCfeatures, where OPC features might include serifs, jogs, and SRAF. Toquantify what is meant by slight variations, a typical slight variationin OPC decoration from neighborhood to neighborhood might be 5% to 80%of a main feature size. Note that for clarity, variations in the designof the OPC are what is being referenced. Manufacturing variations, suchas line-edge roughness and corner rounding, will also be present in theactual surface patterns. When these OPC variations produce substantiallythe same patterns on the wafer, what is meant is that the geometry onthe wafer is targeted to be the same within a specified error, whichdepends on the details of the function that that geometry is designed toperform, e.g., a transistor or a wire. Nevertheless, typicalspecifications are in the 2%-50% of a main feature range. There arenumerous manufacturing factors that also cause variations, but the OPCcomponent of that overall error is often in the range listed. OPC shapessuch as sub-resolution assist features are subject to various designrules, such as a rule based on the size of the smallest feature that canbe transferred to the wafer using optical lithography. Other designrules may come from the mask manufacturing process or, if a characterprojection charged particle beam writing system is used to form thepattern on a reticle, from the stencil manufacturing process. It shouldalso be noted that the accuracy requirement of the SRAF features on themask may be lower than the accuracy requirements for the main featureson the mask.

Inverse lithography technology (ILT) is one type of OPC technique. ILTis a process in which a pattern to be formed on a reticle is directlycomputed from a pattern which is desired to be formed on a substratesuch as a silicon wafer. This may include simulating the opticallithography process in the reverse direction, using the desired patternon the surface as input. ILT-computed reticle patterns may be purelycurvilinear—i.e. completely non-rectilinear—and may include circular,nearly circular, annular, nearly annular, oval and/or nearly ovalpatterns. Since curvilinear patterns are difficult and expensive to formon a reticle using conventional techniques, rectilinear approximationsof the curvilinear patterns may be used. In this disclosure ILT, OPC,source mask optimization (SMO), and computational lithography are termsthat are used interchangeably.

There are a number of technologies used for forming patterns on areticle, including using optical lithography or charged particle beamsystems. Reticle writing for the most advanced technology nodestypically involves multiple passes of charged particle beam writing, aprocess called multi-pass exposure, whereby the given shape on thereticle is written and overwritten. Typically, two to four passes areused to write a reticle to average out precision errors in the chargedparticle beam system, allowing the creation of more accurate photomasks.The total writing time for this type of system increases with the numberof shots. A second type of system that can be used for forming patternson a reticle is a character projection system, which has been describedabove.

The cost of charged particle beam lithography is directly related to thetime required to expose a pattern on a surface, such as a reticle orwafer. Conventionally, the exposure time is related to the number ofshots required to produce the pattern. For the most complex integratedcircuit designs, forming the set of layer patterns, either on a set ofreticles or on a substrate, is a costly and time-consuming process. Itwould therefore be advantageous to be able to reduce the time requiredto form complex patterns, such as curvilinear patterns, on a reticle andother surfaces, such as by reducing the number of shots required to formthese complex patterns.

SUMMARY OF THE DISCLOSURE

A method and system for fracturing or mask data preparation or proximityeffect correction is disclosed, wherein a series of curvilinearcharacter projection shots are determined for a charged particle beamwriter system, such that the set of shots can form a continuous track,possibly of varying width, on a surface. In some embodiments,characteristics of the continuous track will be within a predeterminedtolerance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a character projection charged particle beam system;

FIG. 2A illustrates a pattern and a cross-sectional dosage curveproduced by a single circular CP shot;

FIG. 2B illustrates two proximate, individually-calculated patterns anddosages curves similar to the pattern and dosage curve of FIG. 2A;

FIG. 2C illustrates a pattern and a cross-sectional dosage graph of apair of proximate circular CP shots;

FIG. 3A illustrates a portion of a constant-width target pattern;

FIG. 3B illustrates a set of conventional non-overlapping shots whichcan form the pattern of FIG. 3A;

FIG. 3C illustrates a pattern which can be formed by a single circularCP shot, and also a set of six proximate CP shots;

FIG. 3D illustrates a track which can be formed using the set ofproximate CP shots from FIG. 3C;

FIG. 3E illustrates a set of five proximate CP shots;

FIG. 3F illustrates a track which can be formed using the set of fiveproximate CP shots from FIG. 3E;

FIG. 4A illustrates an example of a target pattern comprising aparallelogram;

FIG. 4B illustrates a pattern that can be formed with a shot of an ovalCP character;

FIG. 4C illustrates a series of seven shots of the same oval CPcharacter as FIG. 4B;

FIG. 4D illustrates a track which can be formed by the set of shots inFIG. 4C;

FIG. 4E illustrates another track which can be formed by the set ofshots in FIG. 4C, using a higher-than-minimum beam blur radius;

FIG. 5A illustrates an example of a curvilinear target pattern;

FIG. 5B illustrates a series of circular CP shots which can form theperimeter of the pattern of FIG. 5A;

FIG. 5C illustrates the pattern formed by the set of perimeter shots inFIG. 5B;

FIG. 6A illustrates a series of three circular CP shots which can form atrack;

FIG. 6B illustrates a series of three CP shots, wherein two of the shotsuse a circular CP character and one of the shots uses an annular CPcharacter;

FIG. 7 illustrates a conceptual flow diagram for manufacturing a reticleand photomask using an exemplary method of the current disclosure; and

FIG. 8 illustrates a conceptual flow diagram for forming a pattern on asubstrate using an exemplary method of the current disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure describes generating and exposing a series ofcurvilinear CP shots to form a continuous track on a surface. A seriesof shots form a spatial succession, and may be written in any temporalorder. Note that the numbers of shots used in the various embodimentsillustrated herein are exemplary only, as a series of shots of thepresent disclosure may number from two or more as desired to achieve thedesired target pattern.

Referring now to the drawings, wherein like numbers refer to like items,FIG. 1 illustrates an embodiment of a conventional lithography system100, such as a charged particle beam writer system, in this case anelectron beam writer system, that employs character projection tomanufacture a surface 130. The electron beam writer system 100 has anelectron beam source 112 that projects an electron beam 114 toward anaperture plate 116. The plate 116 has an aperture 118 formed thereinwhich allows the electron beam 114 to pass. Once the electron beam 114passes through the aperture 118 it is directed or deflected by a systemof lenses (not shown) as electron beam 120 toward another rectangularaperture plate or stencil mask 122. The stencil 122 has formed therein anumber of openings or apertures 124 that define various types ofcharacters 126. Each character 126 formed in the stencil 122 may be usedto form a pattern 148 on a surface 130 of a substrate 132, such as asilicon wafer, a reticle or other substrate. In partial exposure,partial projection, partial character projection, or variable characterprojection, electron beam 120 may be positioned so as to strike orilluminate only a portion of one of the characters 126, thereby forminga pattern 148 that is a subset of character 126. For each character 126that is smaller than the size of the electron beam 120 defined byaperture 118, a blanking area 136, containing no aperture, is designedto be adjacent to the character 126, so as to prevent the electron beam120 from illuminating an unwanted character on stencil 122. An electronbeam 134 emerges from one of the characters 126 and passes through anelectromagnetic or electrostatic reduction lens 138 which reduces thesize of the pattern from the character 126. In commonly availablecharged particle beam writer systems, the reduction factor is between 10and 60. The reduced electron beam 140 emerges from the reduction lens138, and is directed by a series of deflectors 142 onto the surface 130as the pattern 148, which is depicted as being in the shape of theletter “H” corresponding to character 126A. The pattern 148 is reducedin size compared to the character 126A because of the reduction lens138. The pattern 148 is drawn by using one shot of the electron beamsystem 100. This reduces the overall writing time to complete thepattern 148 as compared to using a variable shape beam (VSB) projectionsystem or method. Although one aperture 118 is shown being formed in theplate 116, it is possible that there may be more than one aperture inthe plate 116. Although two plates 116 and 122 are shown in thisexample, there may be only one plate or more than two plates, each platecomprising one or more apertures.

In conventional charged particle beam writer systems the reduction lens138 is calibrated to provide a fixed reduction factor. The reductionlens 138 and/or the deflectors 142 also focus the beam on the plane ofthe surface 130. The size of the surface 130 may be significantly largerthan the maximum beam deflection capability of the deflection plates142. Because of this, patterns are normally written on the surface in aseries of stripes. Each stripe contains a plurality of sub-fields, wherea sub-field is within the beam deflection capability of the deflectionplates 142. The electron beam writer system 100 contains a positioningmechanism 150 to allow positioning the substrate 132 for each of thestripes and sub-fields. In one variation of the conventional chargedparticle beam writer system, the substrate 132 is held stationary whilea sub-field is exposed, after which the positioning mechanism 150 movesthe substrate 132 to the next sub-field position. In another variationof the conventional charged particle beam writer system, the substrate132 moves continuously during the writing process. In this variationinvolving continuous movement, in addition to deflection plates 142,there may be another set of deflection plates (not shown) to move thebeam at the same speed and direction as the substrate 132 is moved.

The minimum size pattern that can be projected with reasonable accuracyonto a surface 130 is limited by a variety of short-range physicaleffects associated with the electron beam writer system 100 and with thesurface 130, which normally comprises a resist coating on the substrate132. These effects include forward scattering, Coulomb effect, andresist diffusion. Beam blur is a term used to include all of theseshort-range effects. The most modern electron beam writer systems canachieve an effective beam blur in the range of 20 nm to 30 nm. Forwardscattering may constitute one quarter to one half of the total beamblur. Modern electron beam writer systems contain numerous mechanisms toreduce each of the constituent pieces of beam blur to a minimum. Someelectron beam writer systems may allow the beam blur to be varied duringthe writing process, from the minimum value available on an electronbeam writing system to one or more larger values.

FIG. 2A illustrates an example of a pattern 202 which would be formed ona resist-coated surface by a shot using a circular CP character. Pattern202 is called a shot outline, which is the pattern that can be formed bythe dosage from a single shot. Throughout this disclosure, a patternwhich is a shot outline may be referred to as a shot, meaning the shotwhich can form the shot outline. Dosage graph 210 illustrates the dosage212 registered along a line 204 through pattern 202, this dosage beingcalled the cross-sectional dosage. As can be seen from dosage curve 212,a pre-determined “full” dosage is registered only in the middle part ofpattern 202. Also shown in dosage graph 210 is the resist threshold 214.The resist will register as a pattern on the surface only those areaswhich receive dosages above the resist threshold 214. The dosage curve212 intersects the threshold 214 at X-coordinates “a” and “b”. TheX-coordinate “a” is therefore the minimum X-coordinate that will beregistered by the resist along line 204, as shown by connector 216.Similarly, the X-coordinate “b” is the maximum X-coordinate that will beregistered by the resist along line 204, as shown by connector 218.

FIG. 2B illustrates an example of two shots in close proximity. Dashedline pattern 220 is the shot outline of a circular CP character shot.Dashed line pattern 222 is the shot outline of another circular CPcharacter shot. The dosage graph 230 illustrates two dosage curves.Dosage curve 232 shows the cross-sectional dosage of shot outline 220,as measured along a line 224. Dosage curve 234 shows the cross-sectionaldosage of shot outline 222, as measured along the line 224. Also shownon dosage graph 230 is the resist threshold 236. As can be seen fromdosage graph 230, the dosage curve 232 and dosage curve 234 overlap,indicating that for some X-coordinates along line 224, the shotsassociated with both shot outline 220 and shot outline 222 willcontribute a measurable dosage. In cases where shot dosage curvesoverlap, the total dosage reaching the resist-covered surface is thecombination, such as by addition, of the dosages from all shotscorresponding to the overlapping curves. FIG. 2C illustrates a dosagegraph 260 which shows the combined dosage curve 262 for shot outline 220and shot outline 222 of FIG. 2B. As can be seen, the combined dosagecurve 262, which shows the dosage along line 224, shows a dosage that isabove the threshold 264 at all X-coordinates between “c” and “d”.Pattern 250 shows the pattern that will be formed on the resist-coveredsurface by the two proximate shots which are associated with shotoutline 220 and shot outline 222. The cross sectional dosage for thispattern is measured along line 254, which corresponds to line 224 ofFIG. 2B. Along line 254, the two proximate shots form a single connectedpattern 250. The left-most intersection of dosage curve 262 withthreshold 264 at X-coordinate “c” determines the minimum X-coordinate ofpattern 250 along line 254, as shown by connector 266. Similarly, theright-most intersection of dosage curve 262 with threshold 264 atX-coordinate “d” determines the maximum X-coordinate of pattern 250, asshown by connector 272. As can be seen, pattern 250 has a non-constantheight in the Y-dimension, due to the use of a circular CP character forshot 220 and shot 222. FIGS. 2B&C illustrate how a plurality ofproximate CP shots of a curvilinear CP character may together produce asingle pattern on a resist-covered surface.

FIG. 3A illustrates an example of a portion of a desired pattern 302 tobe formed on a resist-coated surface. Pattern 302 is a portion of alonger pattern, so the ends of the pattern are not shown. The edges ofpattern 302 are not parallel to either axis of a Cartesian coordinateplane. Pattern 302 may, for example, be part of a metal interconnectlayer on an integrated circuit. Pattern 302 is also a track orcontinuous track, where a track is a pattern that can be visualizedgeometrically as being formed with a single stroke of a paintbrush—i.e.a pattern with no branches. Unlike a normal paintbrush stroke, however,the width of a track may vary along its length. FIG. 3B illustrates theshot outlines 308 of a group of non-overlapping rectangular VSB shots,such as may be conventionally determined for forming pattern 302. Shotgroup 308 shows the shot outlines of 15 shots. The use of conventionalnon-overlapping shots may simplify determination of the pattern that theresist will register from the group of shots. Conventionally, a set ofnon-overlapping shots is determined such that the union of each of theshot outlines will equal the target pattern. The union of the shotoutlines in the group of shots 308 does not quite equal the desiredpattern 302 because the outline of pattern 302 cannot be matched exactlyusing rectangular shots which are oriented parallel to the axes of aCartesian coordinate plane.

FIGS. 3C-3F depict an exemplary method of the present disclosure whereinthe target pattern 302 is formed using a series of curvilinear shots.FIG. 3C illustrates an example of a shot outline 312 using a circular CPcharacter. FIG. 3C also illustrates the shot outlines 314 of a group ofshots which use the same character as pattern 312. The group of shots314 comprises six shots: shot 316, shot 318, shot 320, shot 322, shot324 and shot 326. In this example all the shots in shot group 314 usethe same dosage, but shots using different dosages may also be used toform a track. The group of shots 314 forms a series, because the shotsare in succession spatially, although the shots may be written on thesurface in any temporal order. FIG. 3D illustrates a shape, the shapealso being a track, that may be formed on a surface from the shotsassociated with shot series 314. The overlapping shot outlines in shotseries 314 make the calculation of the resist response, and thereforethe resulting pattern on the surface, more difficult than with shotgroup 308. Charged particle beam simulation may be used to determine thepattern 332 registered by the resist. In one embodiment, chargedparticle beam simulation may be used to calculate the dosage for eachgrid location in a two-dimensional (X and Y) grid, creating a grid ofcalculated dosages called a dosage map. The “wavy” edges in theregistered pattern 332 result from using the spaced circular CPcharacters. The “waviness” of the edges causes variation in the width ofthe pattern 332. The width tolerance for a group of patterns is normallypre-determined. The width variation in pattern 332 can be reduced byspacing the circular CP shots more closely, which will increase thenumber of shots required to form the pattern. Since wider spacing of thecircular CP shots can reduce the shot count and therefore the time towrite the pattern, the pre-determined width tolerance can be used todetermine the maximum acceptable spacing of the circular CP shots. Anadvantage of using circular CP shots for forming target patterns such asthe target pattern 302 is that a circle is radially symmetrical. Theeffects of using a circular CP character are therefore similarirrespective of the angle of the target pattern. The pattern 332illustrates how a series of curvilinear CP shots may be used to form atrack on a surface, where the track is not parallel to an axis of aCartesian coordinate plane.

FIG. 3E illustrates another example of how a series of shots, in thiscase with some overlapping shots, may be used to form a pattern similarto the target pattern 302, using a circular CP character. FIG. 3Eillustrates the shot outlines of a group of shots 340 which use the samecharacter as pattern 312. Shot group 340 comprises five shots: shot 342,shot 344, shot 346, shot 348 and shot 350. As can be seen, the relativespacing of the shots in shot group 340 varies among the shots in thegroup. For example, the spacing between shot 342 and shot 344 is lessthan the spacing between shot 344 and shot 346. Similarly, the spacingbetween shot 350 and shot 348 is less than the spacing between shot 346and shot 348. FIG. 3F illustrates a pattern 360 that may be formed on aresist-covered surface from shot group 340. The waviness of pattern 360varies along its length because of the variable spacing of the shots inshot group 340. For example, the localized minimum width 362 in pattern360 is due to the spacing between shot 342 and shot 344. The localizedminimum width 364 in pattern 360 is due to the spacing between shot 344and shot 346. The relatively larger spacing between shot 344 and shot346 compared to the spacing between shot 342 and shot 344 results in asmaller width 364 compared to width 362. While use of a pre-determinedwidth tolerance will normally suggest that the waviness of a singletrack be consistent to optimize the shot count, the example of FIG. 3Eand FIG. 3F illustrates how a larger shot spacing, with no dosage orbeam blur radius changes, can produce increased waviness in theresulting pattern on the surface. The pre-determined width tolerance forthe final pattern on the surface may therefore be used to determine themaximum acceptable spacing of shots.

Referring again to FIG. 3C, it should be noted that although all shotsin shot series 314 are made using the same character, tracks may beformed using a series of shots using a plurality of characters. In oneembodiment, different sizes of circular CP characters may be used fordifferent subsets of shots in a series of shots, producing a track ofvarying mean width. In another embodiment, a single CP character may beused for all shots in a series, but with different dosages for differentsubsets of shots in the series, also producing a track of varying meanwidth.

FIGS. 4A-4E depict another embodiment of the present invention in whichan oval character is used. FIG. 4A illustrates an example of a desiredpattern or track 402 to be formed on a resist-coated surface. The edgesof track 402 are not parallel to either axis of a Cartesian coordinateplane. Track 402 may, for example, be part of a metal interconnect layeron an integrated circuit. FIG. 4B illustrates a shot outline 404 of anoval CP character. FIG. 4C illustrates the shot outlines of a series ofCP shots 410 using the same oval character associated with shot outline404. Shot series 410 consists of seven overlapping shots: shot 412, shot414, shot 416, shot 418, shot 420, shot 422 and shot 424. As can beseen, the spacing between shot 422 and shot 424 is less than the spacingbetween other pairs of adjacent shots in shot series 410, so as to matchthe length of the track 402. FIG. 4D illustrates a track 430 that may beformed on a resist-coated surface from the shot series 410, using anormal—i.e. minimum—beam blur radius. Like track 332 above, the width oftrack 430 varies along its length. An advantage of using an oval CPcharacter to form the shot series 410, compared to the circular CPcharacter used to form the shot series 314, is that use of an oval shaperesults in a smaller area of overlap between adjacent shots compared touse of a circular shape. The smaller area of overlap between shots inshot series 410 lowers the dosage per unit area compared to shot series314. This may be advantageous by producing a lower level of long rangeeffects such as back scattering when the surface is exposed, compared tothe shot series 314.

FIG. 4E illustrates a track 440 that may be registered by theresist-coated surface from the shot series 410, when ahigher-than-minimum beam blur is used. As can be seen, track 440 issmoother than track 430. Specifically, the difference between themaximum width and the minimum width of track 440 is less than thedifference between the maximum width and the minimum width of track 430.The use of higher-than-minimum beam blur may allow the formation oftracks to a tighter—i.e. smaller—width tolerance than by using theminimum-available beam blur.

A series of curvilinear shots may also be used to form the perimeter ofa pattern, as demonstrated in FIGS. 5A-5C. FIG. 5A illustrates anexample of a curvilinear pattern 502 to be formed on a resist-coatedsurface. The pattern 502 may, for example, be the output of inverselithography processing. The pattern 502 may be described as having fourears—one at each corner. Each ear has a radius of curvature 504, alsomarked “r”. FIG. 5B illustrates the shot outlines of a series 520 oftwelve circular shots that may be used to form the perimeter of pattern502. The radius 524 of the outline of individual shots in the series ofshots 520 is chosen to be “r”, so as to form each of the ears of pattern502 with a minimal shot count. FIG. 5C illustrates a track 540 that maybe produced the series of shots 520. Track 540 is a closed track, withno start or end. The use of circular CP shots allows formation of theperimeter of track 540, which matches the perimeter of pattern 502within a pre-determined tolerance, using fewer shots than if a set ofrectangular VSB shots had been used. Additionally, the use of a circularCP character and dosage which can produce a pattern on the surface witha radius which closely matches the interior radius of a part of thetarget pattern can further reduce the shot count. The series 520 may becombined with additional shots to fill the interior of the pattern 540to achieve the target pattern 502.

FIGS. 6A&B illustrate a comparison of the use of a circular CP characterwith use of an annular CP character in forming a track. FIG. 6Aillustrates the shot outlines for an exemplary series of three shots600, the combination of which will form a track. Shot series 600comprises shot 602, shot 604 and shot 606, all of which are made using acircular CP character. The outline of the resulting track is not shown.Region 608 and region 610 are areas which will receive a dosage above anormal dosage, due to shot overlap. FIG. 6B illustrates the shotoutlines for another exemplary series of three shots 630 which will alsoform a track. Shot series 630 comprises circular shot 612, annular shot614, and circular shot 616. Region 618 and region 620 are theintersecting areas which will receive a dosage above a normal dosage,due to shot overlap. As can be seen, the area of region 618 is less thanthe area of region 608. Similarly, the area of region 620 is less thanthe area of region 610. This smaller area of region 618 compared toregion 608 and region 620 compared to region 610 indicates that lessoverlap dosage will be delivered to the resist-coated surface in shotseries 630 than in shot series 600. The lower dosage of the shot series630 may be preferred so as to produce, for example, a lower level ofbackward scattering than the shot series 600. As is also shown in FIG.6B, region 622, which is part of the “hole” in the outline of annularshot 614, may not register on the resist as a pattern, producing a voidin the resulting track. Since the actual dosage received by any part ofregion 622 is the combination of dosages from shot 612, shot 614 andshot 616, particle beam simulation may be used to determine if thedosage in all parts of region 622 is above the threshold of the resist.If particle beam simulation results show that the dosage in some part ofregion 622 is below the resist threshold, an annular CP character with asmaller hole may be substituted for the annular shot, so that thepattern 622 has a smaller hole. Alternatively, the dosage of anycombination of shots in the pattern may be slightly increased, such asincreasing the dosage for circular shots 612 and 616. In yet otherembodiments, for tracks which are straight tracks, an annular CPcharacter with an elliptical or oval hole may be used, wherein the majoror longer diameter of the hole is aligned with the direction of thetrack. Other more complex shapes may also be used for shot 614. Theexample of FIGS. 6A&B show how the use of annular CP shots may allowformation of tracks with overall lower dosage than circular or othernon-annular curvilinear shots. Careful design can prevent voids in theformed patterns.

Note that curvilinear shapes referred to in this disclosure include butare not limited to circular, nearly circular, oval, nearly oval,elliptical, nearly elliptical, annular, nearly annular, oval-annular,nearly oval-annular, elliptically annular, or nearly ellipticallyannular.

The dosage that would be received by a surface can be calculated andstored as a two-dimensional (X and Y) dosage map called a glyph. Atwo-dimensional dosage map or glyph is a two-dimensional grid ofcalculated dosage values for the vicinity of the shots comprising theglyph. This dosage map or glyph can be stored in a library of glyphs.The glyph library can be used as input during fracturing of the patternsin a design. For example, referring again to FIGS. 4A&C, a dosage mapmay be calculated from the series of shots 410, and stored in the glyphlibrary. If during fracturing, one of the input patterns is a pattern ofthe same shape as pattern 402, then the shots comprising the glyph maybe retrieved from the library, avoiding the computational effort ofdetermining an appropriate set of shots to form input pattern. A seriesof glyphs may also be combined to create a parameterized glyph.Parameters may be discrete or may be continuous. For example, the shotsand dosage maps for forming patterns such as track 402 may be calculatedfor a plurality of pattern lengths, and the plurality of resultingglyphs may be combined to form a parameterized glyph.

FIG. 7 illustrates an exemplary conceptual flow diagram 700 of a methodfor manufacturing a photomask according to the current disclosure. Thereare three types of input data to the process: stencil information 718,which is information about the CP characters on the stencil of thecharged particle beam system; process information 736, which includesinformation such as the resist dosage threshold above which the resistwill register a pattern; and a computer representation of the desiredpattern 716 to be formed on the reticle. In addition, initial optionalsteps 702-712 involve the creation of a library of glyphs. The firststep in the optional creation of a library of glyphs is VSB/CP shotselection 702, in which one or more VSB or CP shots, each shot with aspecific dosage, are combined to create a set of shots 704. The set ofshots 704 may include overlapping VSB shots and/or overlapping CP shots.The set of shots 704 may include a series of curvilinear CP shots whichwill form a track. Shots in the set of shots may also have a beam blurspecified. The VSB/CP shot selection step 702 uses the stencilinformation 718, which includes information about the CP characters thatare available on the stencil. The set of shots 704 is simulated in step706 using charged particle beam simulation to create a dosage map 708 ofthe set of shots. Step 706 may include simulation of various physicalphenomena including forward scattering, resist diffusion, Coulombeffect, etching, fogging, loading, resist charging, and backwardscattering. The result of step 706 is a two-dimensional dosage map 708which represents the combined dosage from the set of shots 704 at eachof the grid positions in the map. The dosage map 708 is called a glyph.In step 710 the information about each of the shots in the set of shots,and the dosage map 708 of this additional glyph is stored a library ofglyphs 712. In one embodiment, a set of glyphs may be combined into atype of glyph called a parameterized glyph.

The required portion of the flow 700 involves creation of a photomask.In step 720 a combined dosage map for the reticle or reticle portion iscalculated. Step 720 uses as input the desired pattern 716 to be formedon the reticle, the process information 736, the stencil information718, and the glyph library 712 if a glyph library has been created. Instep 720 an initial reticle dosage map may be created, into which theshot dosage maps will be combined. Initially, the reticle dosage mapcontains no shot dosage map information. In one embodiment, the gridsquares of the reticle dosage map may be initialized with an estimatedcorrection for long-range effects such as backscattering, fogging, orloading, a term which refers to the effects of localized resistdeveloper depletion. Step 720 may involve VSB/CP shot selection 722, orglyph selection 734, or both of these. Shot selection 722 may compriseselecting a series of curvilinear CP shots which can form a track on thereticle. If a VSB or CP shot is selected, the shot is simulated usingcharged particle beam simulation in step 724 and a dosage map 726 of theshot is created. The charged particle beam simulation may compriseconvolving a shape with a Gaussian. The convolution may be with a binaryfunction of the shape, where the binary function determines whether apoint is inside or outside the shape. The shape may be an aperture shapeor multiple aperture shapes, or a slight modification thereof. In oneembodiment, this simulation may include looking up the results of aprevious simulation of the same shot, such as when using a temporaryshot dosage map cache. A higher-than-minimum beam blur may be specifiedfor the VSB or CP shot. Both VSB and CP shots may be allowed to overlap,and may have varying dosages with respect to each other. If a glyph isselected, the dosage map of the glyph is input from the glyph library.In step 720, the various dosage maps of the shots and/or glyphs arecombined into the reticle dosage map. In one embodiment, the combinationis done by adding the dosages. Using the resulting combined dosage mapand the process information 736 containing resist characteristics, areticle pattern may be calculated. If the calculated reticle patternmatches the desired pattern 716 within a pre-determined tolerance, thena combined shot list 738 is output, containing the determined VSB/CPshots and the shots constituting the selected glyphs. If the calculatedreticle pattern does not match the target pattern 716 within apredetermined tolerance as calculated in step 720, the set of selectedCP shots, VSB shots and/or glyphs is revised, the dosage maps arerecalculated, and the reticle pattern is recalculated. In oneembodiment, the initial set of shots and/or glyphs may be determined ina correct-by-construction method, so that no shot or glyph modificationsare required. In another embodiment, step 720 includes an optimizationtechnique so as to minimize either the total number of shots representedby the selected VSB/CP shots and glyphs, or the total charged particlebeam writing time, or some other parameter. In yet another embodiment,VSB/CP shot selection 722 and glyph selection 734 are performed so as togenerate multiple sets of shots, each of which can form a reticle imagethat matches the desired pattern 716, but at a lower-than-normal dosage,to support multi-pass writing.

The combined shot list 738 comprises the determined list of selected VSBshots, selected CP shots and shots constituting the selected glyphs. Allthe shots in the final shot list 738 include dosage information. Shotsmay also include a beam blur specification. In step 740, proximityeffect correction (PEC) and/or other corrections may be performed orcorrections may be refined from earlier estimates. Thus, step 740 usesthe combined shot list 738 as input and produces a final shot list 742in which the shot dosages have been adjusted. The group of steps fromstep 720 through step 742, or subsets of this group of steps, arecollectively called fracturing or mask data preparation. The final shotlist 742 is used by the charged particle beam system in step 744 toexpose resist with which the reticle has been coated, thereby forming apattern 746 on the resist. In step 748 the resist is developed. Throughfurther processing steps 750 the reticle is transformed into a photomask752.

FIG. 8 illustrates an exemplary conceptual flow diagram 800 of a methodfor manufacturing a substrate such as a silicon wafer according to thecurrent disclosure. There are three types of input data to the process:stencil information 818, which is information about the CP characters onthe stencil of the charged particle beam system; process information836, which includes information such as the resist dosage thresholdabove which the resist will register a pattern; and a computerrepresentation of the desired pattern 816 to be formed on the substrate.In addition, initial optional steps 802-812 involve the creation of alibrary of glyphs. The first step in the optional creation of a libraryof glyphs is VSB/CP shot selection 802, in which one or more VSB or CPshots, each shot with a specific dosage, are combined to create a set ofshots 804. The set of shots 804 may include overlapping VSB shots and/oroverlapping CP shots. The set of shots 804 may include a series ofcurvilinear CP shots which will form a track. Shots in the set of shotsmay also have a beam blur specified. The VSB/CP shot selection step 802uses the stencil information 818, which includes information about theCP characters that are available on the stencil. The set of shots 804 issimulated in step 806 using charged particle beam simulation to create adosage map 808 of the set of shots. Step 806 may include simulation ofvarious physical phenomena including forward scattering, resistdiffusion, Coulomb effect, etching, fogging, loading, resist charging,and backward scattering. The result of step 806 is a two-dimensionaldosage map 808 which represents the combined dosage from the set ofshots 804 at each of the grid positions in the map. The dosage map 808is called a glyph. In step 810 the information about each of the shotsin the set of shots, and the dosage map 808 of this additional glyph isstored a library of glyphs 812. In one embodiment, a set of glyphs maybe combined into a type of glyph called a parameterized glyph.

The required portion of the flow 800 involves creation of a pattern on aresist-covered substrate. In step 820 a combined dosage map for thesubstrate or a portion of the substrate is calculated. Step 820 uses asinput the desired pattern 816 to be formed on the substrate, the processinformation 836, the stencil information 818, and the glyph library 812if a glyph library has been created. In step 820 an initial substratedosage map may be created, into which the shot dosage maps will becombined. Initially, the substrate dosage map contains no shot dosagemap information. In one embodiment, the grid squares of the substratedosage map may be initialized with an estimated correction forlong-range effects such as backscattering, fogging, or loading. Step 820may involve VSB/CP shot selection 822, or glyph selection 834, or bothof these. Shot selection 822 may comprise selecting a series ofcurvilinear CP shots which can form a track on the substrate. If a VSBor CP shot is selected, the shot is simulated using charged particlebeam simulation in step 824 and a dosage map 826 of the shot is created.The charged particle beam simulation may comprise convolving a shapewith a Gaussian. The convolution may be with a binary function of theshape, where the binary function determines whether a point is inside oroutside the shape. The shape may be an aperture shape or multipleaperture shapes, or a slight modification thereof. In one embodiment,this simulation may include looking up the results of a previoussimulation of the same shot, such as when using a temporary shot dosagemap cache. A higher-than-minimum beam blur may be specified for the VSBor CP shot. Both VSB and CP shots may be allowed to overlap, and mayhave varying dosages with respect to each other. If a glyph is selected,the dosage map of the glyph is input from the glyph library. In step820, the various dosage maps of the shots and/or glyphs are combinedinto the substrate dosage map. In one embodiment, the combination isdone by adding the dosages. Using the resulting combined dosage map andthe process information 836 containing resist characteristics, asubstrate pattern may be calculated. If the calculated substrate patternmatches the desired pattern 816 within a pre-determined tolerance, thena combined shot list 838 is output, containing the determined VSB/CPshots and the shots constituting the selected glyphs. If the calculatedsubstrate pattern does not match the target pattern 816 within apredetermined tolerance as calculated in step 820, the set of selectedCP shots, VSB shots and/or glyphs is revised, the dosage maps arerecalculated, and the substrate pattern is recalculated. In oneembodiment, the initial set of shots and/or glyphs may be determined ina correct-by-construction method, so that no shot or glyph modificationsare required. In another embodiment, step 820 includes an optimizationtechnique so as to minimize either the total number of shots representedby the selected VSB/CP shots and glyphs, or the total charged particlebeam writing time, or some other parameter. In yet another embodiment,VSB/CP shot selection 822 and glyph selection 834 are performed so as togenerate multiple sets of shots, each of which can form a reticle imagethat matches the desired pattern 816, but at a lower-than-normal dosage,to support multi-pass writing.

The combined shot list 838 comprises the determined list of selected VSBshots, selected CP shots and shots constituting the selected glyphs. Allthe shots in the final shot list 838 include dosage information. Shotsmay also include a beam blur specification. In step 840, proximityeffect correction (PEC) and/or other corrections may be performed orcorrections may be refined from earlier estimates. Thus, step 840 usesthe combined shot list 838 as input and produces a final shot list 842in which the shot dosages have been adjusted. The group of steps fromstep 820 through step 842, or subsets of this group of steps, arecollectively called fracturing or mask data preparation. The final shotlist 842 is used by the charged particle beam system in step 844 toexpose resist with which the substrate has been coated, thereby forminga pattern 846 on the substrate.

The fracturing, mask data preparation, and proximity effect correctionflows described in this disclosure may be implemented usinggeneral-purpose computers with appropriate computer software ascomputation devices. Due to the large amount of calculations required,multiple computers or processor cores may also be used in parallel. Inone embodiment, the computations may be subdivided into a plurality of2-dimensional geometric regions for one or more computation-intensivesteps in the flow, to support parallel processing. In anotherembodiment, a special-purpose hardware device, either used singly or inmultiples, may be used to perform the computations of one or more stepswith greater speed than using general-purpose computers or processorcores. In one embodiment, the optimization and simulation processesdescribed in this disclosure may include iterative processes of revisingand recalculating possible solutions, so as to minimize either the totalnumber of shots, or the total charged particle beam writing time, orsome other parameter. In another embodiment, an initial set of shots maybe determined in a correct-by-construction method, so that no shotmodifications are required.

While the specification has been described in detail with respect tospecific embodiments, it will be appreciated that those skilled in theart, upon attaining an understanding of the foregoing, may readilyconceive of alterations to, variations of, and equivalents to theseembodiments. These and other modifications and variations to the presentmethods for fracturing, manufacturing a surface, and manufacturing anintegrated circuit may be practiced by those of ordinary skill in theart, without departing from the spirit and scope of the present subjectmatter, which is more particularly set forth in the appended claims.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tobe limiting. Steps can be added to, taken from or modified from thesteps in this specification without deviating from the scope of theinvention. In general, any flowcharts presented are only intended toindicate one possible sequence of basic operations to achieve afunction, and many variations are possible. Thus, it is intended thatthe present subject matter covers such modifications and variations ascome within the scope of the appended claims and their equivalents.

1. A method for fracturing or mask data preparation for charged particlebeam lithography comprising the steps of: inputting a set of characterprojection (CP) characters on a stencil, the set comprising one or morecurvilinear characters; and determining a series of four or more shotsof one or more curvilinear CP characters in the set of CP characters,wherein the series of shots is capable of forming a continuous track ona surface.
 2. The method of claim 1 wherein the series of shotscomprises shots of a plurality of curvilinear CP characters in the setof CP characters.
 3. The method of claim 1 wherein the series of shotscomprises shots of a circular or nearly-circular CP character.
 4. Themethod of claim 3 wherein the series of shots comprises shots of aplurality of circular or nearly-circular CP characters of differentdiameters.
 5. The method of claim 1 wherein the step of determining usesan optimization technique.
 6. The method of claim 5 wherein theoptimization technique reduces the dosage per unit area on the surface.7. The method of claim 5 wherein the optimization technique reduces thetotal writing time for the series of shots.
 8. The method of claim 1wherein each shot in the series of shots comprises a dosage, and whereinthe dosages of the shots in the series of shots vary with respect toeach other before proximity effect correction.
 9. The method of claim 1wherein the step of determining comprises using charged particle beamsimulation.
 10. The method of claim 9 wherein the charged particle beamsimulation includes at least one of the group consisting of forwardscattering, backward scattering, resist diffusion, Coulomb effect,etching, fogging, loading and resist charging.
 11. A method forfracturing or mask data preparation or proximity effect correction forcharacter projection (CP) charged particle beam lithography comprisingthe steps of: inputting a pattern to be formed on a surface, wherein thepattern comprises a continuous track; inputting a set of characterprojection (CP) characters, the set comprising one or more curvilinearcharacters; and determining a series of two or more shots of one or morecurvilinear CP characters in the set of CP characters, wherein theseries of shots is capable of forming the track on the surface, within apre-determined tolerance.
 12. The method of claim 11 wherein thetolerance comprises a track width tolerance.
 13. The method of claim 11wherein the tolerance comprises a track waviness tolerance.
 14. Themethod of claim 11 wherein the input pattern varies in width.
 15. Themethod of claim 11 wherein the number of shots in the series of shots isreduced when compared to use of non-overlapping variable-shaped beam(VSB) shots to form the pattern.
 16. The method of claim 11 wherein theinput pattern is a constant width linear track which is not parallel toeither axis of a Cartesian coordinate plane.
 17. The method of claim 16wherein the one or more curvilinear CP characters used by the shots inthe series of shots are oval, nearly oval, elliptical, nearlyelliptical, annular, nearly annular, oval-annular, nearly oval-annular,elliptically annular, or nearly elliptically annular.
 18. The method ofclaim 17 wherein the waviness, shot count or total dosage for the seriesof shots is reduced when compared to use of circular CP characters forall shots in the series of shots.
 19. A system for fracturing or maskdata preparation for use with shaped beam charged particle beamlithography comprising: an input device capable of receiving a set ofone or more curvilinear character projection (CP) characters; and acomputation device capable of determining a series of two or more shotsof one or more characters in the set of curvilinear CP characters,wherein the series of shots is capable of forming a continuous track ona surface.
 20. The system of claim 19 wherein the series of shotscomprises shots of a circular or nearly-circular CP character.
 21. Thesystem of claim 19 wherein the series of shots comprises shots of aplurality of curvilinear CP characters.
 22. The system of claim 19wherein the computation device uses an optimization technique.
 23. Thesystem of claim 19 wherein each shot in the series of shots comprises adosage, and wherein the dosages of the shots in the series of shots varywith respect to each other before proximity effect correction.
 24. Thesystem of claim 19 wherein the computation device comprises a devicecapable of performing charged particle beam simulation.
 25. The systemof claim 24 wherein the charged particle beam simulation includes atleast one of the group consisting of forward scattering, backwardscattering, resist diffusion, Coulomb effect, etching, fogging, loadingand resist charging.